Electron beam discharge tube having a retarding structure with a tuning device



Sept. 15, 1970 B. B. VAN IPEREN ET AL 3,529,284

. ELECTRON BEAM DISCHARGE TUBE HAVING A RETARDING STRUCTURE WITH A TUNING DEVICE Filed April 15, 1968 3 Sheets-511cc t. 1

' INVENTOR BERNARDUS B. VAN IPEREN WILHELMUS KUYPERS fowl/- Sept. 15, 1970 a. B. VAN IPEREN E Filed April is, 1968 3 Sheets-Sheet I x w 22 25 i E' n i 5; 29 w i 1 Q"; f 28 4 A r f w, v j v v 27 FIGS Y LINVENTOR) asamous 91m IPEREN WILHELMUS xuvesns 2M1; A k

Sept. 15, 1970 B. B. VAN lPEREN ET AL 3,529,204

' ELECTRON BEAM DISCHARGE TUBE HAVING A RETARDING STRUCTURE WITH A TUNING DEVICE] Filed April 15, 1968 s Sheets-sheaf 5 FIG] INVENTOR) BERNARDUS B VAN IPEREN WILHELMUS KUYPERS ACE/ United States Patent Int. Cl. H01j 25/34 U.S. Cl. 315-3.5 2 Claims ABSTRACT OF THE DISCLOSURE An electron beam tube having a retarding structure including at least three substantially identical cavity resonators arranged successively in the direction of length of the tube and coupled electromagnetically, the retarding structure also including a device for tuning the retarding structure which is directly coupled to at least one cavity resonator which is substantially at the center of the series of cavity resonators.

This invention relates to an electron beam tube having a retarding structure comprising a series of at least three substantially identical cavity resonators succeeding in the direction of length of the tube and firmly coupled electromagnetically, leaving a substantially rectilinear longitudinal passage for the electron beam, which retarding structure includes a tuning device intended for tuning the retarding structure as a whole and directly coupled to a fraction of the total number of cavity resonators.

An electron beam tube having a retarding structure comprising a series of electromagnetically-coupled cavity resonators leaving a rectilinear passage for the electron beam is used for amplifying or generating microwaves. Such tubes preferably include means of changing the tuning of the retarding structure, in order to meet with wide tolerances and permit a certain frequency range to be governed. Means such as are sometimes present in tubes intended for meter waves and decimeter waves, which permit each cavity resonator to be tuned individually, become impracticable in tubes intended for the microwave spectrum. The last-mentioned tubes can, however, include a practical means of tuning the structure as a whole. Such tubes having a large number of substantially identical cavity resonators succeeding in the direction of length of the tube and firmly coupled electromagnetically are known in which a tuning device for tuning the structure as a whole is directly coupled to one of the end cavity resonators of the series. At a given point of tuning in such a tube, the energy transmission between the electron beam and the electromagnetic field in the retarding structure is a maximum. Upon changing the tuning from this turning point, the resulting variations in the field will cause a decrease in the energy transmission between the electron beam and the electromagnetic field. When determining a minimum energy transmission required, there is a certain frequency interval within which the tuning can be changed without falling below this minimum. This frequency interval may be referred to as the tuning range. The disadvantage of such a tube is that the energy transmission between the electron beam and the electromagnetic wave varies comparatively rapidly as a function of frequency, in other words the retarding structure has a comparatively small tuning range.

An object of the invention is to provide an electron beam tube having a wider tuning range.

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According to the invention in an electron beam tube having a retarding structure comprising a series of at least three substantially identical cavity resonators succeeding in the direction of length of the tube and firmly coupled electromagnetically, leaving a substantially rectilinear passage for the electron beam, which retarding structure includes a tuning device intended for tuning the retarding structure as a whole and directly coupled to a fraction of the total number of cavity resonators, said tuning device is directly coupled to at least one cavity resonator which is located substantially at the center of the series of cavity resonators.

If the z-direction is chosen as the direction of the path of the electron beam, the energy transmission between the electron beam and the standing electromagnetic waves in the retarding structure in this tube will take place substantially as a result of the interaction between the beam and one of the spatial harmonics of the electric field in the z-direction, for example the m harmonic with a phase constant 13 When the tube is so tuned that a maximum energy transmission occurs between the electron beam and the electromagnetic field, the argument B z of this m harmonic per cavity resonator will increase in the z-direction by a constant value ,fi L if L is the length of a cavity resonator. Upon changing the tuning of the retarding structure over a given frequency band, a variation in field occurs. The m harmonic of the electric field in the z-direction which occurs after a change of tuning will have an argument E z which increases per cavity resonator in the z-direction by a constant amount TJ L which differs from ,B L. In the cavity resonators situated at the ends of the structure, neither of which is directly coupled to the tuning device, the change of tuning will cause an average variation in argument of approximately /2 (E -B )L. If the cavity resonators adjacent the end cavity resonators are neither coupled directly to the tuning device, the average variation therein will be -fl )L. Passing from a cavity resonator situated at the end of the retarding structure towards the nearest cavity resonator to which the tuning device is directly coupled, the variation of the argument in succeeding cavity resonators steadily increases by (F -B )L. In

practical cases B )L is always small so that the field variation in succeeding cavity resonators, passing from a cavity resonator situated at the end of the retarding structure towards the (nearest) cavity resonator to which the tuning device is directly coupled, steadily increases. Consequently, greater field variations occur as the largest number of cavity resonators between one end of the structure and the tuning device is larger, this largest number being minimum if the tuning device is directly coupled to the retarding structure exactly at the center thereof. Thus, when changing the tuning over an equal frequency band, the tube according to the invention exhibits a smaller deformation of field, and hence a smaller variation in transmitted energy, than a tube in which a cavity resonator located at the end of a similar retarding structure is directly coupled to the tuning device. With unchanged requirements with respect to the transmitted energy, the tuning of a tube according to the invention can thus be changed over a larger frequency band, in other words: this tube has a wider tuning range.

The tuning device preferably comprises a wavepipe section transversed to the path of the electron beam and divided by a vacuum window into two parts one of which is directly coupled through a coupling aperture to the cavity resonator(s) situated at the center of the series of cavity resonators and the others of which includes an adjustable tuning member and has at least one aperture so that the tuning device can also be used for coupling to an external Wavepipe circuit.

The invention will now be described, by way of example, with reference to the accompanying drawings.

FIGS. 1, 2, 3, 4 and 5 serve to clarify the occurence of and improvement in the tuning range of a retarding structure comprising succeeding cavity resonators by coupling the tuning device directly to a cavity resonator at the center of the retarding structure, where FIG. 1 schematically shows a known retarding structure comprising a plurality of succeeding identical cavity resonators which cannot be tuned;

FIG. 2 schematically shows a known device comprising the retarding structure of FIG. 1 in which one end cavity resonator is provided with a tuning derive;

FIG. 3 schematically shows a device comprising the retarding structure of FIG. 1 in which, in accordance with the invention, the central cavity resonator is provided with a tuning device;

FIG. 4 is a graph showing a component of the axial electric field at the dots indicated in FIGS. 1, 2 and 3;

FIG. 5 is a graph showing the relationship between the frequency and the phase constants of the waves occurring in the retarding structure of FIG. 1.

FIGS. 6, 7 and 8 show to scale one embodiment of a section of the electron beam tube according to the invention, where FIG. 6 ShOWs a longitudinal section of the electron beam tube;

FIG. 7 is a sectional view of the electron beam tube, taken on the line VIIVII of FIG. 6, and

FIG. 8 is a sectional view taken on the line VIIIVIII of FIG. 7.

Referring now to FIG. 1, a retarding structure 1 comprises a plurality of identical cavity resonators 2 of the reentrant type having nozzles 3 which determine a substantially rectilinear passage for the electron beam through apertures 4. The path of the electron beam extends along the longitudinal axis of the structure, which is indicated as the z-axis. The cavity resonators 2 have coupling slots 5 for their electromagnetic coupling. Dots 6 located at the centers of the cavity resonators 2 are indicated on the z-axis.

The axial resonant electric field in the retarding structure 1 may be written as the sum of. an infinite number of spatial harmonics having phase constants fi which, on the ground of their periodicity, satisfy the relationship:

211% fln 50+T where n is an integer and L is the length of a cavity resonator 2. Furthermore the phase constant ,8 of the fundamental wave satisfies the condition for resonance where k is an integer and N is the total number of cavity resonators. As a rule, the mode is used in which correponding dots in succeeding cavity resonators, such as, for example, the dots 6 indicated in the figure, produce the same field at the same instant. In this case k=0. Net energy transmission between the beam and the field will take place substantially due to the interaction between the beam and the spatial harmonic with phase constant p of which the phase velocity w/p where a: is the frequency, differs a little from the beam velocity.

The retarding structure 1 of FIG. 1 is provided with a piston 7 in FIG. 2 and with a piston 8 in FIG. 3. By means of this piston the tuning of the sturcture may be changed.

If the tuning of the structure 1 is changed by means of an effective reduction in length AL, then a field is produced which satisfies the specified relationship (A) between the phase constants of the spatial harmonics and the said condition for resonance (B), but with the understanding that NL in these relationships has been replaced by NLAL. The phase constant p of the m spatial harmonic in the mode with k=0 varies, on changing the tuning by means of AL, by an amount A,8 depending on AL. The field of the m harmonic at the dots between the end of an end cavity resonator which is not directly coupled to the tuning device then varies by a factor which harmonically depends upon the distance from this end and has a phase constant A,8. From the limiting conditions it follows that said factor must have a maximum at this end.

FIG. 4 graphically shows the influence of a change of tuning by YL on the axial electric field of the m harmonic in the mode with k=0 at the dots 6 indicated in FIGS. 1, 2 and 3. The vertical coordinate indicates the value of the electric field E in the z-direction and the horizontal axis is the z-axis. The dots located on the line I indicate the amplitudes E of the field at the dots 6 of the structure 1 of FIG. 1 the tuning of which has not been changed. These values of the field are asumed simultaneously and vary harmonically in time. The dots located on the full line II indicate the amplitudes of the field at the dots 6 of the structure 1 the tuning of which has been changed by an effective reduction in length AL by means of a device as shown in FIG. 2. These values of the field follow from those indicated by the line I by multiplication by the said harmonic factor with phase constant AB. These values also are assumed simultaneously and vary harmonically in time. The dots on the broken line III indicate the amplitudes of the field at the dots 6 of the structure 1 the tuning of which has been changed by a similar effective variation in length AL by means of a device as shown in FIG. 3. As before, these values are obtained by multiplying by the said harmonic factor. On changing the tuning by the device of FIG. 3, the variation of the field is, as shown in FIG. 4, considerably smaller than that on changing the tuning by a similar effective variation in length by means of the device of FIG. 2. The variation in energy transmission is connected with the total variation of the field. Consequently the tuning range for the device of FIG. 3 is considerably wider than for that of FIG. 2. If, for example,, both devices are to fulfill the condition that, upon changing the tuning, the amplitudes of the field strength at the dots 6 are allowed to decrease not more than up to a fraction fE of E, then the said harmonic factor at the dots 6 must be greater than 1, which may be written as follows:

cos A5 (P /2)Lzf (C) This must hold good for any P, where P indicates the rank number of a cavity resonator when counted from that end cavity resonator which is situated on the same side of the tuning device as the relevant cavity resonator. Since in the device of FIG. 2 the minimum amplitude for the dots 6 occurs for P:7 and in the device of FIG. 3 the minimum amplitude for the dots 6 occurs for P=4,, on the ground of this condition (C) the maximum variation (A6) in the phase constant for the device of FIG. 3 is thirteen-sevenths times as great as the maximum variation (AB) for the device of FIG. 2.

The curve of FIG. 5 shows the relationship between at and {5 for a retarding structure as shown in FIG. 1. The curve is periodic with a period 21r/L. Starting from a phase constant variations AB, which are much smaller than 21r/L, cause variations Aw, which increase more than proportionately as a function of AB. In practical cases the said maximum variations (AM and (Am are as shown in FIG. 5, such small variations, so that the maximum variation (Aw) corresponding to (Am is more than thirteensevenths times as great as the permissible variation (Aw) corresponding to (AM FIG. 6 shows, inside a tube body 11, a retarding structure comprising six identical re-entrant cavity resonators 12 bounded by sleeves 13 and discs 14. The nozzles 15 determine a rectilinear passage for the electron beam, the path of which is indicated by 16. The cavity resonator discs 14 have coupling slots 17. The end cavity resonators are closed by covers 18 and 20, respectively, having apertures 19 and 21, respectively, to allow passage of the electron beam. The central cavity resonators are coupled through an aperture 22 to a wavepipe section 23 having a window 25 and an aperture 27 and provided with a threaded ring 28 for coupling to an external wavepipe circuit. A pin 29 for changing the tuning extends into section 23. FIG. 7 shows that each cavity resonator disc 14 exhibits three coupling slots 17 at positions relatively rotated by 120". The figure also shows that the pin 29 is set in a holder 31 which is pushed against a tuning screw 33 by a spring 32. The tuning screw 33 is screwed into a cover 34 of a housing 35 which is secured to the tube body 11.

The tuning range of the retarding structure in the embodiment above described is approximately four times as large as that in a device having a similar retarding structure in which the tuning device is coupled to one of the end cavity resonators.

An embodiment as above described, which can be constructed in a comparatively simple manner, may be used, for example, as a self-oscillating modulator in a tube for generating sub-millimeter waves.

What is claimed is:

1. An electron beam tube having a retarding structure comprising a series of at least three substantially identical cavity resonators succeeding in the direction of length of the tube, means firmly coupling said cavity resonators electromagnetically, leaving a substantially rectilinear longitudinal passage for an electron beam, and a tuning device for tuning the retarding structure as a whole and directly coupled to a fraction of the total number of cavity resonators, said tuning device being directly coupled to at least one cavity resonator located substantially at the center of the series of cavity resonators.

2. An electron beam tube as claimed in claim 1, wherein the tuning device comprises a wavepipe section transversed to the path of the electron beam and divided by a vacuum window into two parts one of which is directly coupled through a coupling aperture to the cavity resonators of the said fratcion and the other of which includes an adjustable tuning member and has at least one aperture for coupling to an external wavepipe circuit.

References Cited UNITED STATES PATENTS 3,322,997 5/1967 Caryotakis 3155.35 3,360,679 12/1967 Rubert 3153.5 3,441,783 4/1969 Harris et a1 3153.5

HERMAN K. SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner U.S. Cl. X.R. 315-539, 5.46, 5.52

ig g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pat nt No- 3529204 Dated SQpi-Pm'hpr 1a 310 Invent0r(S) BERNARDUS B. VAN OPEREN and WI EIMUS KUYPERS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 12, change "Yp'! to -AL-! (S Anew EdwaralLFletcher, Ir. WIIHIIAM E. SGHUYIIER, JR. LA Offiwr Comissioner of Patent a 

